Radio frequency intrusion detection system

ABSTRACT

In order to detect personnel and other intruding objects in open terrain, one or more long wire antennas are buried in the ground and located orthogonal to a vertically polarized souce of VHF radiation, such as a continuous wave transmitter which feeds an elevated vertical dipole. The intruder couples the radiation to the buried antenna which feeds a receiver. The receiver produces an output responsive to the characteristics of the radiation coupled to the antenna by the intruder to produce an alarm indicative of the presence of the intruder.

United States Patent 1 1 l] 3,794,992 Gehman 1 51 Feb. 26, 1974 15 1RADIO FREQUENCY INTRUSION 3,163,861 12/!964 Suter 343/5 PD x DETECTIONSYSTEM 2,656,527 10/1953 Tillman 340/258 C 3,242,486 3/1966 Corbell343/5 PD [75] Inventor: John B. Gehman, La Jolla, Calif.

[73] Assignee: General Dynamics Corporation, St. PrimaryExaminer-Malcolm F. Hubler Louis, Mo. Attorney, Agent, or FirmMartinLukacher [22] Filed: Feb. 7, 1972 B T T [211 App]. No.: 224,130 [57] A 8RM:

In order to detect personnel and other intruding ob- I jects in openterrain, one or more long wire antennas [52] 343/5 340/258 340/258 C areburied in the ground and located orthogonal to a [51] Int. Cl G01s 9/02,(i08b 13/26 vertically polarized souce of VHF radiation Such as a [58]held of 340/258 258 343/5 PD continuous wave transmitter which feeds anelevated vertical dipole. The intruder couples the radiation to [56]References C'ted the buried antenna which feeds a receiver. The re-UNITED STATES PATENTS ceiver produces an output responsive to thecharacter- 3.727,208 4/1973 Anderson et a1 340/258 C istics of theradiation coupled to the antenna by the 3,497,995 3/1970 Forsberg v340/258 C X intruder to produce an alarm indicative of the pres- KleistCt C X ence of the intruder 2,455,376 12/1948 Lindsay 340/258 C3,237,105 2/1966 Kalmus 340/258 c 24 Claims, 12 Drawing Figures Y/4 0\BSA BSA m BSA /28 36 40 I6 MONITORING 48 AND POWERN I2 DISTRIBUTIO#TRANSMITTING SYSTEM ANTENNA 0 VHF 5 l0 TRANS- 52* R MITTE W BSA 44BURlED SENSING ANTENNA (BSA) PATENTED 3794.992

SHEI 1 BF 3 BSA 33 m 40 as MONIT N6 AND P wen o RIB T I E' "TRANSMITTINGl2 ANTENNA VHF 50 TRANS- l0 MITTER [8 34 so 22 42 I 7 BSA 44 m INCIDENTSIGNAL Q ,r A ,4 \WCOAXIAL CABLE 22 y SURFACE 1'0 L5 FT 4 :2 \f TOPOWERAND\ k\\\\ ABLE\] PAIIZNIED 3.794.992

xntEI 2 OF 3 DISTANCE TO BSA Lima I ALARM F 4 BSA INDICATOR 66 F765 i WW a2 a4 FROM 86 RECVR. faa TO ALARM 76 INDICATOR f 94 92 COMPARATOR ANDRECVR. BSA] .90 mm *W BSAg ALARM F RECVR BSA, INDICATOR PAIENTED FEB26I974 SHEET 3 BF 3 RADIO FREQUENCY INTRUSION DETECTION SYSTEM The presentinvention relates to security systems, and particularly to a system forsensing personnel and other intruding objects through the use of radiofrequency electromagnetic wave radiation.

The invention is especially suitable for use in security systems forprotecting the boundaries of a large area in open terrain by sensing thepresence of personnel and other intruding objects crossing theseboundaries.

Electromagnetic personnel sensing systems of various types have beensuggested. For the most part they operate in accordance with principlesof capacitive and inductive coupling between the personnel or otherobject to be detected and a sensing device (see for example U.S. Pat.Nos. 3,439,358 and 3,462,692). Other systems operate on the basis of thedisturbance in a standing wave pattern produced by intruding personnel(see U.S. Pat. No. 2,038,878). Still others operate in a manner similarto radar systems and detect reflections from the intruding personnel(see U.S. Pat. 3,163,861). While it may be feasible to utilize suchsystems to secure relatively limited areas, they are not particularlyadapted for securing a large area, say in open terrain, such as theperimeter of a factory or other large installation. In addition suchknown systems may be difficult to install, particularly in a manner suchthat they are not obtrusive to intruding personnel who may be interestedin avoiding or disabling the sensing system.

It is therefore, an object of the present invention to provide animproved electronic security system.

It is a still further object of the invention to provide an improvedsecurity system which utilizes radio frequency electromagneticradiation.

It is a still further object of the invention to provide an improvedradio frequency security system which is sensitive to moving objectssuch as personnel.

It is a still further object of the invention to provide an improvedradio frequency security system which is especially adapted for securinglarge areas, out of doors and in open terrain.

It is a still further object of the invention to provide an improvedradio frequency security system which may readily be installed at lowcost.

It is a still further object of the present invention to provide animproved radio frequency security system which is unobtrusive and theoperation of which is not readily ascertainable by intruding personnel.

Briefly described, the present invention affords a system for detectingthe presence of an object, such as personnel, in a radio frequency,radiation field, through the use of an antenna disposed below thesurface of the terrain upon which the radiation field is incident. Theantenna may be a long wire, say 50 to 400 feet in length, connected to areceiver which detects the radio frequency signal which is coupled tothe antenna by intruding personnel. The intruding personnel act as areceiving antenna or reflector and provide the necessary coupling to theburied sensing antenna. The intruder will then cause a variation in thefield strength of the signal monitored by the receiver, such that thereceiver produces an output indicative of the presence of the intruder.

The invention itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof willbecome more readily apparent from a reading of the following descriptionin connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a radio frequency object detectionsystem in accordance with the invention, which system is especiallyadapted for securing the perimeter of a large area;

FIG. 2 is a perspective view, schematically illustrating the operationof the system in sensing intruding personnel;

FIGS. 3a, 3b, and 3c are schematic diagrams illustrative of theoperation of radio frequency security systems embodying the invention;

FIG. 4 is a wave form illustrative of the output from a receiver of thesystem shown in FIGS. 1 and 2 which results when intruding personnelpass over the buried sensing antenna;

FIG. 5 is a block diagram of the receiver portion of a system embodyingthe invention;

FIG. 6 is a schematic diagram of circuits used in the receiver portionof a system embodying the invention;

FIG. 7 is a block diagram illustrating a radio frequency objectdetection system in accordance with another embodiment of the invention;

FIG. 8 is a block diagram of a system in accordance with still anotherembodiment of the invention;

FIG. 9 is a more detailed diagram, partially in block form and partiallyin schematic form, which illustrates a receiver system in accordancewith an embodiment of the invention; and

FIG. 10 is a block diagram of a monitoring system suitable for use witha number of receivers of the type illustrated in FIG. 9, whichmonitoring system is provided in accordance with features of the presentinvention.

The system provided by the invention, unlike systems requiringelectromagnetic (either inductive or capacitive) coupling to a sensingdevice or direct reflection of signals to a pick-up antenna, isespecially adapted to protecting large areas in open terrain. Thus, whena large area, such as several acres is to be secured, several buriedsensing antennas, e.g., six antennas l0, l2, 14, 16, 18 and 20, may bedisposed about the perimeter of the area to be secured.

Each of these antennas may be a length of insulated wire (No. 10 to No.30, wire being suitable) which are disposed in linear paths about theperimeter of the area and buried below the surface of the ground.Perferably, the sensing antennas are buried to a depth of one to threeinches. They may, however, be buried up to 18 inches below the ground.The antennas are connected in the center thereof, via coaxial cables,22, 24, 26, 28, 30 and 32, to receivers 34, 36, 38, 40, 42 and 44. Thecenter connection reduces losses due to attenuation in the wire of theantennas. The receivers may, however, be connected to one end of theirrespective antenna wires. The receivers are preferably sensitive narrowbandwidth radio receivers of the type to be described hereinafter inconnection with FIG. 9. However, any receiver capable of detectingsignals in the frequency range of the radio frequency radiation fieldestablished in the vicinity of the antennas may be used. The receivers,34, 36, 38, 40, 42 and 44 may also be buried underground, and receivepower and transmit alarm signals through a power distribution and alarmline or cable, 46. This cable, 46, is connected to a monitoring andpower distribution system 48 which translates the alarm outputs from thereceivers into visual or aural indications, say an array of lights,which designates which antenna through 20, has sensed the presence of anintruding object or person.

The radio frequency radiation is established by a transmitting antenna50 which preferably is a vertical dipole which transmits a verticallypolarized radiation field. The antenna 50 is driven by a VHF continuouswave transmitter 52. Frequencies between 40 MHz and ISO MHz arepreferable. For personnel detection a frequency of 60 MHZ is especiallypreferred since a man is approximately one quarter wavelength long atabout 60 MHz, and thus acts as a good receiving antenna or reflector totranslate the radio frequency radiation from the transmitting antennainto the buried sensing antennas.

It will be noted that the buried sensing antennas are disposedorthogonally to the transmitting antenna 50 rejecting the directvertically polarized signal. However, the vertically polarized signalfrom the transmitting antenna reflected or coupled to a vertical targetsuch as a person, is especially well picked up by the buried horizontalsensing antenna. The vertical target is believed to cause ground currentto flow near the buried sensing antenna wire, which in turn induces thesignal into the wire. It may also reflect a signal along the axis of theantenna which has a maximum response. In other words, the intrudingpersonnel provides coupling to the wire of the buried sensing antennaand will increase the magnitude of the signal picked up from the remotetransmitter by reflection. Thus the receivers 34 through 44 coupled totheir respective buried sensing antennas, produce outputs which vary inaccordance with the field strength of the signal picked up by the buriedsensing antennas. When an intruder approaches a buried sensing antenna,this field strength increases and is detected in the receivers toindicate the presence of the intruder.

The operation of the system may be better understood from FIG. 2. Theburied sensing antenna, 10, is shown by way of example. It is indicatedas being 100 feet long. With normal ground conditions (the ground notbeing highly conductive) the buried wire of the antenna 10 can be 400feet long. The antenna is indicated as being buried up to a depth of 1.5feet and is connected by way of the buried coaxial cable to the receiver34 which is also buried. The cable to the power and monitoring station46 is also shown as being buried.

Very little signal may normally be received by the buried sensingantenna 10 due to its orthogonal relationship with respect to thetransmitting antenna and because the transmitting radiation isvertically polarized, while the buried antenna is disposed horizontally.The intruder 4 acts like a vertical reflector and directly couples theradio frequency signal directly into the antenna 10. In the event thatthe antenna 10 picks up some of the incident signal, the signal coupledto the antenna by the intruder will add vectorially to the signal whichis normally picked up. The approach of the intruder will then cause acyclical variation in the output from the receiver 34 as is illustratedby way of example in FIG. 4. In effect, a doppler signal is generated asthe intruder moves toward the sensing wire 10 if he moves in thedirection toward the signal source (viz., the antenna 50). A cycle isgenerated for every half wavelength of motion at the frequency of theradiation.

When the intruder crosses the sensing wire still in the direction towardthe If source a much lower frequency doppler signal is generated. Thissignal characteristic is also illustrated in FIG. 4. Thus, by viewingthe signal from the receiver, say on the chart recorder, the presence ofan intruder can be visually ascertained. The receiver may also have athreshold detector, as will be discussed in detail in connection withFIG. 5 through 9 for automatically indicating the presence of theintruder.

The radiation pattern of the antenna 10 is also illustrated in FIG. 3a.It will be noted that the antenna has a bidirectional pattern centeredabout the center point 56 of the antenna wire to which the receiver 34is connected. The antenna pattern has a multiplicity of lobes and favorsvertically polarized signals coming at an angle above the ground andalong the axis of the wire. Accordingly, vertically polarized signalsfrom the transmitter reflected from the intruder are picked up by thehorizontal wire antenna 10.

In FIG. 3, one of the lobes (shown in the dash line) of an antenna wire,58, which is connected to a receiver 60 at one end thereof, is shown. Itwill be understood that a plurality of such lobes, the number of whichdepends on the length of the wire, will exist. The radiation patternfavors somewhat an intruder near either end of the sensing wire antenna58. Pickup of radiation signals by a buried horizontal sensing antenna,such as shown at 62 in FIG. 30, is also effected by the passage of theintruder above the center of the antenna causing ground currents(illustrated schematically by the lines, 64 and 66) to flow near theantenna 62 and induce signals therein. It is believed that signals arecaused to be picked up due to the presence of an intruding object by oneor more of the mechanisms discussed, in connection with FIGS. 2 and 3,although the invention is not limited to any theory by which pickup ofradiation sig' nals may be explained, which explanation is offeredherein for purposes of elucidation.

As illustrated in FIG. 5 automatic alarm indication may be afforded byconnecting the output of a receiver 66 which receives input signals fromthe buried sensing antenna by way of a capacitor 68 to a comparatoramplifier 70. In the receiver the signal, such as illustrated in FIG. 4,which is picked up by the buried sensing antenna and applied to theinput of the receiver, is detected to produce a slowly varying analogsignal corresponding thereto which is coupled to the comparator 70 viathe capacitor 68. The comparator receives a threshold level from apotentiometer, 72. When this threshold is exceeded by the amplitude ofthe signal from the receiver output, the comparator produces a pulsewhich triggers an alarm indicator 74. This indicator may, for example,by a latching relay which illuminates a lamp or sounds a buzzer or otheraudible alarm. The alarm may be reset manually.

Inasmuch as the output from the receiver may vary cyclically, it may becoupled, as shown in FIG. 6, through a capacitor 76 and oppositelypolarized diode rectifiers 78 and 80 to the direct and inverting inputsof an operational amplifier 82. The rectifier and operational amplifieract as a full wave rectifier and amplifier translating the cyclic inputfrom the receiver into a unipolar output which is coupled through acapacitor 84 to a comparator 86. The comparator 86 has a thresholdvoltage applied to another input thereof from a potentiometer 88, thusthe full wave rectified output exceeds the threshold. The comparator 86provides an output pulse to an alarm indicator, such as discussed abovein connection with FIG. 5.

The motion of the intruder produces a cyclical response when movingtoward the buried sensor antenna or on a parallel path. This response isdue to the vector addition of direct signal leakage or miscellaneousscattered reflections from fixed objects and the reflected signal fromthe moving intruder. The positive or negative peaks are in orout-of-phase conditions, zero responses are quadrative additions. Toreduce the possibility of a missed detection due to a chance path givinga quadrature addition, zero response, two buried sensor antennas BSA,and BSA are used as indicated in FIGS. 7 and 8. One of the antennas hasan added quarterwavelength section, or a delay line 90 which provides aquadrative shift of the received signal with respect to the other buriedsensing antenna. An electronic switch 92 which may be a pair of circuitgates alternately enabled by the output of a free running multivibratoris used to switch the outputs from the antenna alternatively to theinput of a receiver 94. One or the other or both will provide thereceiver with a peak signal regardless of any path the intruder takes.

In FIG. 8 instead of an electronic switching circuit 22, a pair ofreceivers 100 and 102 are separately connected to the buried sensingantennas BSA and BSA the receiver 102 being connected by way of thedelay line 90, from BSA The outputs of the receivers are capacitivecoupled by capacitors 103, and 105 and are additively combined by way ofa diode network 104 and 106. The rectified signals from receivers 100and 102 are connected to the input of a comparator 108. Positive outputfrom 104 and 106 is applied to the noninverting input of 108 while thenegative output from 104 and 106 is applied to the inverting input ofcomparator 108. The connection via the resistor pads 111 is additiveinto comparator 108. A threshold voltage for the comparator 108 isobtained from a potentiometer 110. The change in output from thecomparator which results when the threshold is exceeded, operates analarm indicator 112. Again, in the case of FIG. 8, the quadrature andin-phase signals resulting from the intruder are additively combined inthe circuits 104 and 106 which are connected to the outputs of thereceivers 101 and 102, thus providing an output from an intrusionregardless of the path taken by the intruder.

Since the signal expected to be produced when an intruder is present hasa very slow variation (viz., of the order of l Hz or less), thebandwidth of the receiver is desirably narrow. Such narrow bandwidthaids in increasing sensitivity and reducing the adverse effects of noiseand interference. The receiver shown in FIG. 9 provides the necessarynarrow bandwidth and noise discriminating characteristics as well asother features. The signal from the buried sensing antenna is firstamplified in a radio frequency amplifier 114, and then is applied to afrequency translator including a first mixer 116. This first mixer maybe connected to two local oscillators 118 and 120. These two localoscillators may be alternatively selected so as to provide two channels,say for radiation at two different frequencies, e.g., 100 MHz and 105MHz. The channels may be selected by alternating the polarity of thesupply voltage from the monitoring station by means of a double poledswitch 122. Since the lines of the power distribution cable, which areof opposite polarity are connected separately to the local oscillators118 and 120 through different diodes 124 and 126 by way of chokes 128and 130, a different one of the oscillators will be operating at any onetime depending upon the position of the switch 122 and the polarizationof the diodes. Thus, either channel may be remotely selected from themonitoring station.

After amplification in an intermediate frequency amplifier 140, a secondstage of frequency translation is provided in a second mixer 142 whichreceives injection signals from a second local oscillator 144. In orderto afford a degree of narrow bandwidth the intermediate frequencyamplifier 146 which selects the mixer product at the mixer 142 may havea narrow bandwidth of approximately 500 Hz. The intermediate frequencysignals from the amplifier 146 may be detected in an automatic gaincontrol amplifier 148 and applied by way of a gain control potentiometer150 to the RF and IF amplifiers 114 and 140. The output of the IFamplifier 146 is applied to a phase lock loop 152 including a phasediscriminator 154 and a low-pass filter 156 having a 1H2 bandwidth whichcontrols a voltage controlled variable frequency oscillator 158. Theloop 152 thus remains locked to the frequency of the radiation receivedby the sensing antenna. The received signal is applied to a synchronousdetector 160 together with a signal from the voltage control oscillator158, shifted 90 by a phase shift circuit 162. The phase lock loop, phaseshift, synchronous detector and integrator combination affords anadditional degree of filtering which narrows the bandwidth to ll-Izreducing the possibility of external interference, and/or audiomodulation on the sensing carrier. The double conversion in the receiverintermediate frequency stages also aids in increasing the sensitivity ofthe system. The output of the synchronous detector which is integratedin the integrator circuit 164 is applied to a rectifier and thresholddetector circuit 166 such as shown in FIG. 6, which changes its leveland triggers a one-shot multivibrator 168. The multivibrator 168 enablesan oscillator to produce a burst of oscillation for the period of thepulse in the one-shot multivibrator, say two seconds in duration, thefrequency of the oscillations from the oscillator 170 is indicated asf,,. Suitably, this frequency may be any frequency in a band say, from100 to 500 KHz. The tone burst from the oscillator, 170, is coupled viacapacitors 172 and 174 to the power distribution and alarm line, whenceit is delivered to terminals 176 and 178 at the monitoring station.

The power distributed along the line is coupled by way of chokes 180 and182 to a bridge rectifier 184. The output of the bridge rectifier isvoltage regulated in a voltage regulator 186 which may be a zener diodeand produces operating voltages indicated at V and -V which are appliedto the circuit components of the receiver for operating these componentsand the entire receiver from voltages generated at the monitoringstation and distributed along the line.

A system for displaying the alarm output from several receivers isillustrated in FIG. 10. The signals are applied to terminals 176 and 178and capacitively coupled to a wideband amplifier 188. The widebandamplifier can thus handle and amplify various identification frequenciesproduced by oscillators, such as the oscillator 170 in the receivershown in FIG. 9. The output of the amplifier is applied to a mixer 190which receives injection signals from a variable frequency voltagecontrolled oscillator 192. The frequency at which this oscillatoroperates is controlled by a digital to analog converter 194 whichreceives a digital signal from a recycling counter 196. The counter isdriven by a clock oscillator 198 which causes the counter to count, sayto 10. recycling every second, for example. Since the number reached bythe counter varies as each new clock pulse arrives at the oscillator198, the voltage developed by the converter 194 changes correspondinglyas does the injection from the voltage controlled oscillator 192.Accordingly, the frequency at which input signals will provide mixerproducts which will pass through the bandwidth of an intermediatefrequency amplifier 200 will vary with each count. Thus, during eachcycle of the counter operation, each of the possi ble identificationfrequencies produced by oscillators, such as the oscillator 170 in theseveral receivers which may be included in the system, will be sampled.The output of the amplifier 200 is detected and amplified in adetectorand amplifier circuit 202 which may drive a speaker to producean audible alarm if any of the receivers detect an intruder. A visualalarm will be provided at an indicator lamp A through J, each of whichcorresponds to a different tone burst frequency. Thus, AND gates 206 to208, each corresponding to a different one of the lamps A through J,will be successively enabled by the counter 196. These gates will beenabled in time relationship with the different expected identificationbursts. If an AND gate 206 to 208 is enabled, a flip-flop 210 associatedtherewith will be set and current will flow through one of the lamps Athrough J. The flip-flops may be reset manually through a pushbuttonswitch 212. Since there will be a corresponding time relationship forthe enabling of the gates 206 to 208 and the arrival of detected outputscorresponding to different frequency tone bursts, each of the indicatorlamps will correspond to a different identifcation burst and thereforeto a different receiver and its associate sensing antenna. A panoramicdisplay is therefore provided which facilitates the monitoring ofseveral sensing antennas in the system.

From the foregoing description it will be apparent that there has beenprovided an improved object detection system which utilizes radiofrequency radiation for the detection of personnel and other intrudingobjects. While the system has been described in connection with atransmitter and antenna for providing the radiation which is picked upby the sensing antenna, it will be appreciated that the system may alsoutilize existing sources of signals as may be broadcast by broadcaststations in the vicinity, especially FM broadcast stations. Othervariations and modifications within the scope of the invention willundoubtedly suggest themselves to those skilled in the art. Accordingly,the foregoing description should be taken merely as illustrative and notin any limiting sense.

What is claimed is:

1. A radio frequency intrusion detection system for detecting thepresence of an intruding object in a radio frequency radiation fieldadapted to be produced by a radio frequency transmitting source, whichcomprises a receiving antenna disposed remotely from the transmittingsource of said radiation field at such a distance that the capacitativeand inductive coupling from the source is insignificant at the radiationfield frequency, said antenna being disposed below the surface of theterrain upon which said radiation field is incident and to which saidfield is coupled when said object is in proximity to said antenna, andreceiver means connected to said antenna and responsive to radiofrequency signals generated in said antenna by said radiation field whencoupled to said antenna by said object for providing an outputindicative of the presence of said object.

2. The invention as set forth in claim 1 wherein said terrain is theground, and said antenna is buried below the surface of said ground adistance less than the penetration depth for the frequency of saidradiation field.

3. The invention as set forth in claim 2 wherein said radiation field isof a frequency in the VHF band and said distance is less than 1 foot.

4. The invention as set forth in claim 1 wherein said antenna is a longwire.

5. The invention as set forth in claim 4 wherein said receiver isconnected to one end of said wire.

6. The invention as set forth in claim 4 wherein said receiver isconnected to the center of said wire.

7. The invention as set forth in claim 4 wherein said wire is at least50 feet in length.

8. The invention as set forth in claim 5 wherein said wire is from 50 to400 feet in length from the point where it is connected to said receiverto an end thereof.

9. The invention as set forth in claim 4 wherein said wire is disposedorthogonally to the direction of propagation of said radio frequencyradiation field.

10. The invention as set forth in claim 1 wherein said system is adaptedto secure an area and wherein a plurality of said antennas are disposedin end to end relationship about the perimeter of said area.

11. The invention as set forth in claim 10 including a plurality ofreceivers separately connected to different ones of said antennas.

12. The invention as set forth in claim 11 including common monitoringmeans connected to all of said re ceivers for detecting the outputs ofsaid receivers indicating the presence of an object in the proximity ofeach of said antennas.

13. The invention as set forth in claim 1 wherein said antenna is a longstraight wire, and including a transmitting antenna for radiating wavespolarized vertically with respect to said wire to establish saidradiation field.

14. The invention as set forth in claim 13 wherein said transmittingantenna is a whip antenna disposed vertically upon said surface andlocated remotely from said wire antenna.

15. The invention as set forth in claim 1 including a second antennadisposed in side-by-side relationship with said first named antennameans for providing said second antenna with a length approximatelyonequarter wavelength different in length from said first antenna, andmeans for connecting said first and second antenna to said receiver.

16. The invention as set forth in claim 15 wherein said connecting meanscomprises switch means for connecting said first and second antennas inrapid succession to said receiver.

17. The invention as set forth in claim 15 wherein said receivercomprises first and second receivers connected respectively to saidfirst and second antennas, each of said receivers providing a separateoutput and means responsive to the sum of said separate outputs forproviding said indication of the presence of said object.

l8. The invention as set forth in claim 1 wherein said receiver includesa phase lock loop having a narrow band pass.

19. The invention as set forth in claim 18 wherein said phase lock loopcomprises a phase discriminator to which a signal corresponding to theradiation detected by said antenna is applied and a variable frequencyoscillator and a narrow band pass filter for applying the output of saiddiscriminator to said oscillator for controlling the frequency thereof,a synchronous detector, responsive to said signal, and means forshifting the phase of the oscillations from said oscillator and applyingsaid shifted oscillation to said synchronous detector, and meansresponsive to the output of said synchronous detector for providing saidoutput indicative of the presence of said output.

20. The invention as set forth in claim 18 wherein said receiver furtherincludes a frequency translator for heterodyning the signal detected bysaid antenna to an intermediate frequency signal and means for applyingsaid intermediate frequency signal to said phase lock loop.

21. The invention as set forth in claim 20 wherein said frequencytranslator includes a mixer, first and second local oscillatorsconnected to said mixer, a direct current power supply line forproviding operating voltage to said receiver and means for selectivelyreversing the polarity of direct current applied to said line forselectively enabling said first and second local oscillators.

22. The invention as set forth in claim 18 wherein said receiver furtherincludes means responsive to said receiver output for translating saidoutput into an identification burst of predetermined frequency andduration which corresponds to the reception of said output.

23. The invention as set forth in claim 22 wherein said system comprisesmonitoring means responsive to said tone bursts which have differentfrequencies over a band of frequencies, said monitoring means includinga wide band amplifier for amplifying said band of frequencies, a mixercoupled to the output of said amplifier, a variable frequency oscillatoralso coupled to said mixer, detector means coupled to said mixer forproviding signals corresponding to said tone bursts which are convertedby said mixer to a given frequency, a recycling counter providing aplurality of outputs corresponding to different counts, means forapplying clock pulses to said counter causing it to count, a digital toanalog converter connected to said counter for converting the countsstored in said counter into an analog signal and applying said analogsignals to said oscillator for varying the frequency thereofcorrespondingly with said counter, and means operated by said counterfor separately displaying'said detector means output signals which occursimultaneously with different ones of said counts.

24. The invention as set forth in claim 6 wherein said wire is from 50to 400 feet in length from the point where it is connected to saidreceiver to an end thereof.

1. A radio frequency intrusion detection system for detecting thepresence of an intruding object in a radio frequency radiation fieldadapted to be produced by a radio frequency transmitting source, whichcomprises a receiving antenna disposed remotely from the transmittingsource of said radiation field at such a distance that the capacitativeand inductive coupling from the source is insignificant at the radiationfield frequency, said antenna being disposed below the surface of theterrain upon which said radiation field is incident and to which saidfield is coupled when said object is in proximity to said antenna, andreceiver means connected to said antenna and responsive to radiofrequency signals generated in said antenna by said radiation field whencoupled to said antenna by said object for providing an outputindicative of the presence of said object.
 2. The invention as set forthin claim 1 wherein said terrain is the ground, and said antenna isburied below the surface of said ground a distance less than thepenetration depth for the frequency of said radiation field.
 3. Theinvention as set forth in claim 2 wherein said radiation field is of afrequency in the VHF band and said distance is less than 1 foot.
 4. Theinvention as set forth in claim 1 wherein said antenna is a long wire.5. The invention as set forth in claim 4 wherein said receiver isconnected to one end of said wire.
 6. The invention as set forth inclaim 4 wherein said receiver is connected to the center of said wire.7. The invention as set forth in claim 4 wherein said wire is at least50 feet in length.
 8. The invention as set forth in claim 5 wherein saidwire is from 50 to 400 feet in length from the point where it isconnected to said receiver to an end thereof.
 9. The invention as setforth in claim 4 wherein said wire is disposed orthogonally to thedirection of propagation of said radio frequency radiation field. 10.The invention as set forth in claim 1 wherein said system is adapted tosecure an area and wherein a plurality of said antennas are disposed inend to end relationship about the perimeter of said area.
 11. Theinvention as set forth in claim 10 including a plurality of receiversseparately connected to different ones of said antennas.
 12. Theinvention as set forth in claim 11 including common monitoring meansconnected to all of said receivers for detecting the outputs of saidreceivers indicating the presence of an object in the prOximity of eachof said antennas.
 13. The invention as set forth in claim 1 wherein saidantenna is a long straight wire, and including a transmitting antennafor radiating waves polarized vertically with respect to said wire toestablish said radiation field.
 14. The invention as set forth in claim13 wherein said transmitting antenna is a whip antenna disposedvertically upon said surface and located remotely from said wireantenna.
 15. The invention as set forth in claim 1 including a secondantenna disposed in side-by-side relationship with said first namedantenna means for providing said second antenna with a lengthapproximately one-quarter wavelength different in length from said firstantenna, and means for connecting said first and second antenna to saidreceiver.
 16. The invention as set forth in claim 15 wherein saidconnecting means comprises switch means for connecting said first andsecond antennas in rapid succession to said receiver.
 17. The inventionas set forth in claim 15 wherein said receiver comprises first andsecond receivers connected respectively to said first and secondantennas, each of said receivers providing a separate output and meansresponsive to the sum of said separate outputs for providing saidindication of the presence of said object.
 18. The invention as setforth in claim 1 wherein said receiver includes a phase lock loop havinga narrow band pass.
 19. The invention as set forth in claim 18 whereinsaid phase lock loop comprises a phase discriminator to which a signalcorresponding to the radiation detected by said antenna is applied and avariable frequency oscillator and a narrow band pass filter for applyingthe output of said discriminator to said oscillator for controlling thefrequency thereof, a synchronous detector, responsive to said signal,and means for shifting the phase of the oscillations from saidoscillator and applying said shifted oscillation to said synchronousdetector, and means responsive to the output of said synchronousdetector for providing said output indicative of the presence of saidoutput.
 20. The invention as set forth in claim 18 wherein said receiverfurther includes a frequency translator for heterodyning the signaldetected by said antenna to an intermediate frequency signal and meansfor applying said intermediate frequency signal to said phase lock loop.21. The invention as set forth in claim 20 wherein said frequencytranslator includes a mixer, first and second local oscillatorsconnected to said mixer, a direct current power supply line forproviding operating voltage to said receiver and means for selectivelyreversing the polarity of direct current applied to said line forselectively enabling said first and second local oscillators.
 22. Theinvention as set forth in claim 18 wherein said receiver furtherincludes means responsive to said receiver output for translating saidoutput into an identification burst of predetermined frequency andduration which corresponds to the reception of said output.
 23. Theinvention as set forth in claim 22 wherein said system comprisesmonitoring means responsive to said tone bursts which have differentfrequencies over a band of frequencies, said monitoring means includinga wide band amplifier for amplifying said band of frequencies, a mixercoupled to the output of said amplifier, a variable frequency oscillatoralso coupled to said mixer, detector means coupled to said mixer forproviding signals corresponding to said tone bursts which are convertedby said mixer to a given frequency, a recycling counter providing aplurality of outputs corresponding to different counts, means forapplying clock pulses to said counter causing it to count, a digital toanalog converter connected to said counter for converting the countsstored in said counter into an analog signal and applying said analogsignals to said oscillator for varying the frequency thereofcorrespondingly with said counter, and means operated by said counterfoR separately displaying said detector means output signals which occursimultaneously with different ones of said counts.
 24. The invention asset forth in claim 6 wherein said wire is from 50 to 400 feet in lengthfrom the point where it is connected to said receiver to an end thereof.